Hold on a minute, all the "gut instinct" flag waving goes against so many posts on here - someone comes on with a feeling that their DH is playing away, or "feeling funny" about someone and get roundly told to get a grip or give their head a wobble. Health concerns - you probably have health anxiety.
Of course knife crime - any crime - is a concern, and legitimate concerns based on evidence should be reported.
But living in a world where we routinely police people and their actions based on their looks tips us into paranoia and will erode any hope of doing what really needs to be done to tackle problems such as knife crime.
We often see posts on here defending young disenfranchised men, but if we view them all with suspicion you'll play into their sense of being aggrieved and potentially add to their alienation.
Being constantly viewed as a threat or a danger is really unpleasant.
And targeting people in this way can get really out of hand - remember the college professor that was under suspicion of the murder of his tenant? He was innocent but it didn't stop him being arrested and smeared all over the papers based on his perceived eccentricity and not much else.
If the lad in this situation had been heard talking on his phone saying he was going to shank his dealer - fine. If he'd been muttering threats under his breath - fine. But being dressed "casually" and smelling of weed isn't enough in my opinion. And like some others, I very much doubt trotting into a shop with CCTV and age policies and legit ID is the modus operandi of someone intent on a crime.
People invested in knives for collections have their own sources, and generic kitchen knives are available in most homes.
My point is, we already live in divisive and judgemental times, and this kind of thing doesn't help anyone's mental health or community.
I do feel for the OP, but this sort of thing is a really sad indictment of the times we live in all round.
I have adult sons and I still worry about them if they're out in town of a night, but I don't want to ban the sale of knives when there are policies in place already to limit harm and age limits etc. It will all just very out of hand and restrictive.